1. Field of the Invention
The present invention relates to a reflective guest-host liquid crystal display device. More particularly, the present invention relates to a technique for improving the utility efficiency of the incident light by providing a quarter wavelength plate (.lambda./4 phase shifter) and an optical reflective layer within the device. Moreover, the present invention particularly relates to a technique to increase the brightness of the display by diffusely and efficiently emanating the reflected light.
2. Description of the Related Art
Although liquid crystal devices according to various modes are available, Tn modes or STN modes respectively employing twist oriented or super-twist oriented nematic liquid crystals are predominantly used in these days. However, it is difficult to achieve a bright display according to these modes, the working principle of which requires one pair of polarizing plates absorbing light, resulting in low transmittance. In addition to the above TN and STN modes, guest-host modes utilizing dichroic dyes have been developed. Guest-host liquid crystal display devices display images by utilizing the anisotropy of the absorption coefficient of the dichroic dyes added to the liquid crystal. When an electric field is applied to a liquid crystal, which employs a dichroic dye having a long-cylindrical structure, so as to alter the orientation direction of the liquid crystal molecules, the orientation direction of the dye molecules also changes because they are aligned along the liquid crystal molecules. Since the coloring of the dye changes depending on the orientation direction, the colored state and the colorless state of the liquid crystal display can be switched by applying a voltage.
FIG. 6 shows a structure of a Heilmeier-type guest-host liquid crystal display device. FIG. 6A illustrates a state of a liquid crystal to which no voltage is applied, and FIG. 6B indicates another state of the liquid crystal to which a voltage is applied. This liquid crystal display device employs a p-type dye and a nematic liquid crystal (N.sub.p liquid crystal) which exhibits positive dielectric anisotropy. Since the p-type dichroic dye has an absorbance axis substantially parallel to the molecular axis, it strongly absorbs the polarization component Lx which is parallel to the molecular axis, and scarcely absorbs the polarization component Ly which is perpendicular to the polarization component Lx. In the liquid crystal shown in FIG. 6A to which no voltage is applied, the polarization component Lx included in the incident light is strongly absorbed in the p-type dye, and the display is thereby colored. Meanwhile, in the liquid crystal shown in FIG. 6B to which a voltage is applied, the molecules of the N.sub.p nematic liquid crystal having positive dielectric anisotropy are oriented perpendicular to the direction of the incident light in response to the applied electric field, and the molecules of the p-type dye are thereby aligned to the same direction as that of the liquid crystal molecules. Therefore, only a very small quantity of the polarization component Lx is absorbed in the dye and the display becomes substantially colorless. Another polarization component Ly included in the incident light is scarcely absorbed in the dichroic dye, regardless of whether a voltage is applied to the liquid crystal or not. Thus, in the Heilmeier-type guest-host liquid crystal display device, the polarization component Ly is removed by setting up a polarizing plate to improve the contrast of the resultant display.
In guest-host liquid crystal display devices employing the nematic liquid crystal, the dichroic dye added to the liquid crystal as a guest is oriented in a similar direction to the nematic liquid crystal, and it absorbs the polarization component which is parallel to the orientation direction of the liquid crystal and scarcely absorbs another polarization component which is perpendicular thereto. Therefore, for providing sufficient contrast for the display, a polarizing plate is set up in the incident side of the liquid crystal display device such that the incident light is polarized to the orientation direction of the liquid crystal. However, according to this method, 50% (in practice approximately 40%) of the incident light is lost in principle because of the polarizing plate, resulting in a dark display similarly to that of a TN mode. If the polarizing plate is simply removed, the absorbance ratio of the "on" state to the "off" state extremely decreases. Various kinds of improvements have been proposed to solve the above problems. For example, as is shown in FIG. 7, a reflective guest-host liquid crystal display device is proposed which has a quarter wavelength plate and a reflector at the outgoing side and which does not have a polarizing plate at the incident side. According to this system, the directions of the polarization components Lx and Ly in the incident path, which components are perpendicular to each other, are rotated at 90.degree. C. in the reflection path by the quarter wavelength plate and replaced by each other. Thus, in the "off" state shown in FIG. 7A (i.e., absorption state), each of the polarization components Lx and Ly is absorbed during the incident path or reflection path. Meanwhile, in the "on" state shown in FIG. 7B (i.e., transmission state), both of the polarization components Lx and Ly are scarcely absorbed during the incident path and reflection path. Therefore, the utility efficiency of the incident light is significantly improved, resulting in a brighter display.
Since the quarter wavelength plate and the reflector are installed outside the device, a transmission liquid crystal display device must be employed for the above structure. In particular, when an active matrix structure is used for displaying moving pictures with excellent resolution, thin-film transistors for driving pixel electrodes are integrally formed on a substrate. Thus a certain part of the incident light is cut off in the transmission liquid crystal display devices because of the small aperture ratio of the pixels. Therefore, it is impossible to achieve significantly brighter display, even if the polarizing plate is removed from the device. FIG. 8 shows an example of the structure of liquid crystal display devices which have the quarter wavelength plates and reflectors inside the devices to solve the above problems. The display device shown in FIG. 8 is disclosed in Ser. No. 08/629,637 filed on Apr. 9, 1996 and Ser. No. 08/684,299 filed on Jul. 17, 1996. These applications are to be assigned to the assignee of the present application and incorporated in the present application as a reference.
In the present invention, the structure having a quarter wavelength plate and a reflector inside the device is referred to as "guest-host liquid crystal display device with quarter wavelength plate". As is shown in FIG. 8, the guest-host liquid crystal display device with quarter wavelength plate is composed of a first substrate 101 positioned at the side of incident light 100 and a second substrate 102 positioned behind the first substrate 101 with a predetermined space therebetween. In this space, a guest-host liquid crystal layer 103 is arranged at the side of the first substrate 101 and a quarter wavelength plate 104 is positioned at the side of the second substrate 102. Further, for applying a voltage to the guest-host liquid crystal layer 103, electrode layers 105 and 106 are provided at the side of the first substrate 101 and at the side of the second substrate 102, respectively. Furthermore, an optical reflective layer 107 is integrally formed with the electrode layer 106 at the side of the second substrate 102. The optical reflective layer 107 is provided between the second substrate 102 and the quarter wavelength plate 104 so as to substantially specularly reflect the incident light 100 and emanate reflected light 108. A passivation layer 109 is formed between the liquid crystal layer 103 and the quarter wavelength plate 104.
In general, for achieving brighter display, the optical reflective layer 107 and the electrode layer 106 are integrated, and reflective electrodes having the maximum aperture ratio, such as aluminum films, are used in the reflective guest-host liquid crystal display devices. However, since flat metal-film electrodes cause specular reflection, the visual angle is extremely restricted and a display with a metallic tone instead of a paper-white tone is obtained. For preventing the above phenomenon, it is proposed to finely roughen the surface of the optical reflective layer, which is made of a metal film, to increase the distribution of the reflection angles. According to this method, a process for roughening is required. Further, in the guest-host liquid crystal display device with quarter wavelength plate, the quarter wavelength plate 104 is required to be formed accurately along the roughness at a uniform thickness, which requirement is practically very difficult to fulfil. Furthermore, it is necessary to control the distribution of the inclination angles of the roughness for achieving an optimum visual angle, which fact does not fit with reality. As above mentioned, in the manufacture process of liquid crystal devices, numerous problems occur to finely roughen the surface of the optical reflective layer, i.e. a metal film.
In general, a metallic deposited film, such as an aluminum film, is formed as an optical reflective layer of a display device having a built-in reflector. Since this optical reflective layer is a substantially specular reflector, it specularly reflects the incident light and exhibits high directivity. Therefore, relating to the external illumination light, the brightness of the display extremely varies according to the visual angle and it becomes hard to see the display. In addition, when the optical reflective layer is almost a specular reflector, it exhibits a metallic tone which is not always suitable for a background of the display.